Rotary wing aircraft generally includes a rotor hub that supports rotor blades as well as various rotor blade control elements. The rotor blades, rotor blade control elements, and other structure which, when rotated, cause vibration. Currently, many rotor hubs are provided with bifilar vibration absorbers. A bifilar vibration absorber includes a support and one or more sets of bifilar masses each including a dynamic mass and two tuning pins. The tuning pins radially constrain the dynamic mass. The dynamic mass is free to move relative to the rotor hub. The support is fixedly attached to the rotor hub.
During operation, the dynamic mass, acting as a pendulum, moves in a pendulous movement relative to the support. When the natural frequency of the mass is equal to a rotor excitation frequency, vibrations are either reduced or eliminated. Currently, the bifilar vibration absorber is positioned to reduce and/or eliminate vibrations in a plane defined by rotor blade rotation. That is, rotation of the mass in a plane of the rotor blade rotation develops an opposing force that reduces rotor excitations that may exist in the plane of rotor blade rotation. Vibrations may still exist along a plane that is substantially normal to the plane of rotor blade rotation.